Fix: Type hinting for Python <3.9

[PROxZIMA]

Description

Fixes #29

Motivation and Context

Adds backward compatibility.

How Has This Been Tested?

$ pyenv install 3.8.2
$ pyenv global 3.8.2
$ python -m unittest -b

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.